1. Field of the Invention
The present invention relates a magnetic recording medium, such as a magnetic tape, magnetic sheet, or the like, and a process of fabricating such magnetic recording medium.
2. Description of the Prior Art
In recent years, as magnetic materials, particularly recording mediums used for the video tape recorder and computer are improved for higher recording density and for higher S/N ratio, magnetic powder of smaller grain size is used. For example, if considered in term of the BET value that refers to the surface area per unit weight, magnetic powder of a class of 19 to 23 m.sup.2 /g is used for the standard grade of the magnetic tape of both VHS and BETA system; magnetic powder of a class of 23 to 28 m.sup.2 /g for the higher grade of the magnetic tape; and magnetic powder of a class of 29 to 33 m.sup.2 /g for the superhigh grade of magnetic tape. Further, in the near future, powder of a class of 35 to 50 m.sup.2 /g is expected to marketed.
Generally, the S/N ratio of a magnetic recording medium is believed to be proportional to the square root of the number of grains of magnetic material in the recording medium that are involved in recording and playback. When the same weight of magnetic powder is used for coating, therefore, use of a magnetic material of a smaller grain size is more preferable in increasing the S/N ratio. Since the surface area of grains is inversely proportional to the square of the grain size, however, a decreasing grain size means sharply increasing difficulties encountered in the dispersion of powder and more deterioration in the stability of dispersion.
Normally, to disperse magnetic powder, a quantity of dispersant that is just enough to cover the surfaces of all grains of magnetic powder must be sufficient. Actually, however, it does not give satisfactory dispersion and stability and, therefore, the dispersant is added in considerable excess. The fraction of dispersant that is not adsorbed on the surfaces of magnetic grains mixes with the binder in the coating, which plasticizes the magnetic layer or prevents hardening of the binder, resulting in a pronounced reduction in the mechanical strength of the magnetic layer and particularly in the Young's modulus. Recently, with improvements of the magnetic tape for longer recording, there appears a tendency to use a thinner base film to achieve a smaller total thickness of tape. Since the stiffness of the tape is proportional to the cube of the tape thickness, the tape loses its stiffness sharply as it is made thinner. This results in inferior running performance of the thin tape and its poor head contact, leading to a lowered S/N ratio. To maintain the mechanical properties, and particularly the stiffness of tape in spite of a smaller thickness, a super-oriented base film is adopted and the magnetic layer is improved in the Young's modulus. For this reason, a dispersant in excess and other low molecular additives, if used, impair the mechanical properties of a thin film significantly.
Various techniques have been disclosed to disperse the magnetic powder effectively and stably without affecting the mechanical properties of the magnetic layer adversely. For example, in Japanese Patent Opening Nos. 94308/1979, 143894/1979 and 92103/1975, the magnetic powder is pretreated with a phosphate ester derivative.
Further, in Japanese Patent Opening Nos. 134899/1976, 51703/1978, 7898/1978 and 46509/1979, there is disclosed a technique of coating the surface of magnetic layer with silicone oil.
Further, in Japanese Patent Opening Nos. 108902/1975, 97738/1974, 33753/1976, 116114/1978, 24000/1979, etc., a surface treatment with an anionic surfactant is made. However, these disclosures can not be asserted to be effective in case of magnetic powder of smaller grains and particularly the one having a BET value of 35 to 40 m.sup.2 /g or over.
Further, in Japanese Patent Opening Nos. 103403/1976, 33602/1972, 125169/1980, 73929/1980, 73930/1980, 42888/1982, and 1026/1982, techniques are disclosed to coat magnetic grains with oligomer or polymer containing functional groups that can be adsorbed on the surface of these grains.
These techniques mix dry magnetic grains with a dispersant solution to have the dispersant adsorbed on the surface of these grains in a process that requires successive steps of dissolving the dispersant, mixing with magnetic grains, agitation, kneading, filtration, drying, pulverization, and sieving. Since dry magnetic powder is provided in a coagulated state, however, addition of a dispersant thereto and kneading of the mixture will fail to disperse the magnetic powder so fully as to separate individual primary grains of such powder, making it difficult to coat individual primary grains with polymer. It is noted that forcible dispersion until these primary grains separate will break needle-like grains. Clusters, each containing a plurality of magnetic grains, are thus coated with polymer. For this reason, no improvement in the dispersibility is achieved.
Among various kinds of magnetic powder, particularly, Co-deposited .gamma.-Fe.sub.2 O.sub.3 powder is widely used for the magnetic material of video tape, high fidelity audio tape and computer tape. The general process of preparing such Co-deposited .gamma.-Fe.sub.2 O.sub.3 is as follows. First, sodium hydroxide (NaOH) is added to a solution of ferrous sulfate (FeSO.sub.4.7H.sub.2 O) to produce ferrous hydroxide (Fe(OH).sub.2), which is oxidized in air and washed to form .alpha.-FeOOH for use as nuclei for crystalline growth. Another solution of ferrous sulfate is prepared, to which .alpha.-FeOOH as nuclei and metal iron are added. As the solution is heated under air bubbling, iron is oxidized and dissolves to form .alpha.-FeOOH, which deposits in surfaces of .alpha.-FeOOH that presents as nuclei for crystalline growth. As crystals, for example, 0.6 to 1.0.mu. long and 0.1 to 0.3.mu. thick have grown, the reaction is stopped. Filtration, washing with water and drying give powder of geothite. This is further dehydrated and reduced to Fe.sub.3 O.sub.4 in a H.sub.2 stream at a temperature of about 400.degree. C., which is converted to .gamma.-Fe.sub.2 O.sub.3 as it is slowly reheated up to about 200.degree. C. .gamma.-Fe.sub.2 O.sub.3 thus prepared is used to provide nuclei for further crystalline growth. It is introduced in an aqueous solution of iron sulfate and cobalt sulfate. Addition of an alkali followed by 1 to 2 hour oxidation at 60.degree. to 80.degree. C. then deposits cobalt iron oxide in surfaces of .gamma.-Fe.sub.2 O.sub.3 nuclei. Filtration, washing with water and drying gives Co-deposited Fe.sub.2 O.sub.3 grains.
A technique that is concerned in such Co-deposited .gamma.-Fe.sub.2 O.sub.3 powder is disclosed, for example, in Japanese Patent Opening No. 138110/1982 wherein in the process of preparing magnetic powder a cationic surfactant is added at the step of washing with water that follows the step of cobalt deposition to increase the squareness ratio and improve packing. However, even such technique has a problem that the dispersibility is still unsatisfactory and a favorable squareness ratio, etc. cannot be achieved. Further, in Japanese Patent Opening No. 10903/1981, a technique is disclosed that Co-deposited .gamma.-Fe.sub.2 O.sub.3 that is still at a wet state is treated with low molecular surfactant. However, even this technique fails to give satisfactory dispersibility.
On the one hand, as a treatment agent to treat magnetic powder, Japanese Patent Opening Nos. 23207/1975 and 22603/1975 disclosed a kind of copolymer. It contains monomer units of ##STR1## Its original carboxyl groups are partially converted to amido groups (--CONHR) with an aim that their alkyl group (--R) may give a lipophilic property so in kneading the magnetic paint the magnetic powder may be more miscible with the binder. Therefore, in a process that includes a treatment in an aqueous system like the process of preparing the Co-deposited .gamma.-Fe.sub.2 O.sub.3 as mentioned above, it is impossible to surface treat magnetic grains with the above copolymer without causing poor dispersion of magnetic grains in the aqueous medium.
The present authors have reached a conclusion that though the process of preparing the magnetic powder (for example, Co-deposited .gamma.-Fe.sub.2 O.sub.3) its primary magnetic grains are separately dispersed in the wet state after their preparation, but that once dried, these grains are clustered so firmly that it is very difficult to redisperse them separately as before and particularly needle-like magnetic grains, if forcibly redispersed separately, are liable to breakage, etc. As a result, a surface treatment of magnetic powder at a clustered state occurs, which has no effect in improving.